The present invention relates to a method and apparatus for detecting decrease in tire air-pressure, and a program for judging decompression of a tire. More particularly, it relates to a method and apparatus for detecting decrease in tire air-pressure, and a program for judging decompression of a tire with which it is possible to detect simultaneous decompression of two wheel tires of both front wheels or rear wheels.
An apparatus for detecting decrease in tire air-pressure (DWS) in which decompression of a tire is detected on the basis of rotational (wheel speed) information of four wheel tires attached to a vehicle is conventionally known. Such an apparatus employs a theory that a rotational velocity or a rotational angular velocity of a compressed tire is increased when compared to remaining normal tires owing to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire of normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires (reference should be made to Japanese Unexamined Patent Publication No. 305011/1988),DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is employed as a judged value DEL. Here, F1 to F4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
Since the tires are manufactured to include variations (initial differences) within standards, effective rolling radii of the respective tires are not necessarily identical even though all of the tires are at normal internal pressure. This will result in variations in the rotational angular velocities of the respective tires. To cope with this point, there is suggested a technique in, for instance, Japanese Unexamined Patent Publication No. 206460/1998, in which initial correction coefficients for eliminating differences in effective rolling radii affecting rotational angular velocities are obtained whereupon the velocities of the respective tires are corrected by using such initial correction coefficients so as to perform judgment of decompression of tires on the basis of the judged values DEL.
However, since decompression is judged on the basis of a difference in sums of rotational angular velocities of the four wheels which are respectively located diagonally with respect to each other, it is impossible to simultaneously detect decompression of both front wheels or both rear wheels. It therefore exists the danger that fuel consumption is worsened through increase in rolling resistance of the tire and that a burst might be caused upon continuing running without being aware of decompression.